In the case of an eye with emmetropia (normal vision), an object at infinity will be imaged on the retina located at the rear of the eyeball. In an eye having poor vision, such an object is imaged on the retina unclearly in the form of dispersion circles.
In the case of myopia (nearsightedness), a distant object is sharply imaged in front of the retina. This is caused by the fact that a myopic eye either has a length which is too large in relationship to the refractive power of the normal eye or too high a refractive power in relationship to the length of the normal eye. Too high a refractive power of the eye is caused by the cornea of the eye having too great a curvature.
In the case of hyperopia (farsightedness), a remote object is sharply imaged behind the retina. In this situation, either the length of the eye is too short or the refractive power of the eye is too low.
Especially in the case of persons born with myopia, attempts have been made to reduce the refractive power of the eye with the aid of refractive corneal surgery which reduces the curvature of the cornea.
One known solution is radial keratotomy wherein up to 16 radial cuts are made in the cornea of varying length and depth. This leads to a flattening of the central corneal region which effects a correction of 2 to 6 diopters. A correction achieved by the operation is as a rule substantially reduced in the first few months thereafter.
From German published patent application DE-OS No. 32 15 832, it is also known to make a circular incision in the cornea of the eye concentric to the optical zone which too leads to a central flattening of the cornea. A circular insert is placed in this incision which holds the cornea in its flattened position.
A microkeratom was developed by J. I. Barraquer and is shown in FIG. 1 of the drawing and, with the aid of which, it is possible to cut off a corneal lamella parallel to the surface of the cornea. In this connection, reference may be made to an article by J. I. Barraquer in "Archivos De La Sociedad Americana Oftalmologia" Volume 6, (1967) pages 69 to 101. This microkeratom is made up of a base ring which is seated upon the eye to be operated and fixed there in position with the aid of a partial vacuum. A support is guidably mounted for movement on the base ring and moves an oscillating knife across the eye. The device includes an applanate surface against which the corneal surface lies during the movement of the knife. When the oscillating knife is moved across the eye lying against the applanate surface, a lamella is cut off which is parallel to the surface of the cornea lying against the applanate surface.
The corneal lamella separated in this manner is frozen pursuant to the method which is known as keratomileusis and, in this condition, is processed on a rotating worktable for frozen materials. For example, it can be thinned at the central region and then sewn onto the patient's eye. This process is imprecise because changes in the volume of the corneal lamella can occur in the freezing process. The process has also the medical disadvantage that the previously frozen corneal lamella can cause long-lasting irritations of the corneal surface.
With keratophakia, lens tissue from donor corneal material is placed between the cornea and the corneal lamella which was surgically removed from the patient's eye. The lamella is then secured. Here too, the donor cornea is processed in the frozen condition.
The same is true also for epikeratophakia wherein a lamella-shaped lens made from donor cornea is placed upon the cornea of the patient's eye and sewn thereon after the epithelium is removed.
With all such surgical methods, an interlamellar corneal scar is formed which is dependent upon the quality of the corneal cut.